Foamed rubber compositions for pneumatic tires and method of producing the same

ABSTRACT

A foamed rubber composition for pneumatic tires comprises at least one diene polymer as a rubber ingredient and includes closed cells in a matrix rubber, each of these cells being covered with a coat layer made from a given amount of a resin or resin composite having a JIS-C hardness of not less than 75, a particle size of 10-200 μm and a reacted conjugate diene unit content of not less than 10% by weight. In the production of the foamed rubber composition, the melting point or glass transition point of the resin or resin composite is restricted to at least 5° C. lower than the vulcanizing temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to novel foamed rubber compositions for pneumatictires and a method of producing the same, and more particularly to afoamed rubber composition suitable for use in a tread of a pneumatictire, in which each of the resulting closed cells in a matrix rubber iscovered with a coat layer, and a method of producing the same.

2. Description of Related Art

Recently, the demand for so-called all season tires capable of usingwithout a tire exchange even in winter season likewise summer season isincreasing. This type of the all season tire develops dry grippingproperty, wet gripping property, steering stability, durability and lowfuel consumption even in winter season likewise summer season, andfurther has sufficient traction and braking performances on snow and/orice roads.

As a tread rubber used in the all season tire, there have hitherto beenknown a method of decreasing a hardness at a low temperature in a treadrubber for summer season, a method of using a polymer having a low glasstransition temperature or using a softening agent capable of adequatelyholding a modulus of elasticity of the tread rubber at a lowtemperature, and the like.

In the former method, however, the performances at snow and icetemperature range are almost developed owing to the hysteresis propertyof the polymer, but there are problems that the braking performance andsteering stability on wet and dry roads are insufficient. On the otherhand, the latter method is disclosed, for example, in JP-A-55-135149,JP-A-58-199203, JP-A-60-137945 and the like, but has a serious problemthat it badly affects the wear resistance and durability in the runningon general-purpose road though the performances on snow and/or ice roadsare improved.

In any case, the tread rubbers produced by the above methods certainlyexhibit good performances at a relatively low temperature range of nothigher than -5° C. or at so-called dry-on-ice state, but do not providesufficient friction coefficient at a wet state of about 0° C. or atso-called wet-on-ice state, so that it can not be said that the tractionperformance, braking performance and steering stability are sufficientlyimproved.

Lately, there are adopted some processes of applying means for theimprovement of friction force to the tread rubber composition itself. Asa first process, there is a process wherein the tread rubber is foamedby a proper means to form closed cells in the tread rubber composition(JP-A-63-89547). In general, a layer called as a false liquid layer isexistent on a surface of an ice. When an object is forcedly moved on theice surface, a part of the false liquid layer changes into a water film,which acts as a lubricant to develop a low friction constant. In thefirst process, the surface of the thus obtained tread rubber are coveredwith a great number of cells, so that the removal of water film createdon a zone contacting with ice surface and the edge action of scrapingthe false liquid layer accompanied with a microscopic movement of thecell are developed to provide high friction properties on ice. The tiresmanufactured by using this process are commercially available as astudless tire. However, they have the effect of removing the falseliquid layer from the ice surface, but can not directly scrape or breakthe ice itself different from the conventionally used spike tire, sothat the improvement of the performances on ice is not yet satisfied.

As a second process, there is a process for attaining the high frictionproperties on ice by incorporating various high-hardness materials intothe tread rubber to utilize the scratching effect of these materials onice surface (JP-B-46-31732, JP-A-51-147803, JP-B-56-52057 andJP-B-6-102737). This process is apparent to be a process for providingthe high friction properties of the tread rubber on ice by a mechanismdifferent from the above first process. In fact, as the amount of thehigh-hardness material incorporated becomes larger, the tread rubbertends to provide the high friction properties on ice.

In the first process, the surface of the tread rubber is rendered into arugged surface by foaming, so that ice surface is scratched by convexportions of the rugged surface and water existing on ice surface isabsorbed and discharged by concave portions of the rugged surface.Moreover, the water absorbing and discharging effect is not required ata low temperature range hardly melting the ice surface (not higher than-3° C. in usual case), but the scratching effect can not be expected toomuch at such a low temperature range.

In the second process of incorporating the high-hardness materialdeveloping a high scratching effect into the matrix rubber of the tread,the effect of improving the performances on ice at about 0° C. having alarge water content is small at this temperature. Further, thehigh-hardness material is existent as a foreign matter having noaffinity with rubber, so that the wear resistance and fatigue propertiesare considerably degraded.

Under the above circumstances, the inventor has examined theincorporation of composite particle consisting of syn-1,2-polybutadieneresin, sulfur, vulcanization accelerator, carbon black and scorchretarder into a foamed tread rubber in order to simultaneously establishthe improvement of performances on snow and/or ice road surfaces at wetstate and the wear resistance and confirmed that the use of such acomposite particle is not yet sufficient in a level required forcommercial markets. That is, it is demanded to further improve thecomposite particle.

Moreover, the actual ice surface temperature variously changes over atime of from day to night, so that it is strongly desired to develop atire tread indicating more stable performances on ice at a widertemperature range and causing no degradation of wear resistance andfatigue properties.

On the other hand, the conventional foamed rubber has a function ofimproving the friction coefficient on ice by the dewatering and waterdischarging effects of flowing out water film generated between a groundcontact region of the tire and an ice road surface. However, sucheffects are critical in order to remove a greater amount of water filmat the wet-on-ice state, so that it is difficult to improve the frictioncoefficient on ice at the wet-on-ice state.

Alternatively, the performances on ice are improved to a certain extentby incorporating particles having particular particle size and hardnessas microspike for developing the scratching effect into the treadrubber, but the synergistic action between the scratching effect and thewater discharging effect is less, so that the level of the performanceson ice is still insufficient. That is, the feature that the synergisticaction is small is due to the fact that the hardness of the particle isin inverse proportion to the adhesion property of the particle torubber. In the conventional technique, as the particle becomes harder,the adhesion property of the particle to rubber lowers though thescratching effect is improved. As a result, the particles are apt to befallen out from the ground contact region of the tread at a frictionface between the tire and the road surface and hence the number ofeffective microspikes undesirably reduces and the scratching effect isnot held long and also there are problems that the wear resistance andcrack resistance are degraded. When the hard particles are added to arubber composition containing a foaming agent, the foaming is caused atboundary surfaces of the particles to lower the adhesion force to matrixrubber.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a novel foamedrubber composition for pneumatic tires exhibiting a friction coefficienton ice at not only dry-on-ice state but also wet-on-ice state enough todevelop a true synergistic action between the scratching effect and thewater discharging effect at maximum as well as a method of producing thesame.

According to a first aspect of the invention, there is the provision ofa foamed rubber composition for pneumatic tires comprising at least onediene polymer selected from the group consisting of natural rubber,isoprene rubber, styrene-butadiene copolymer rubber, butadiene rubberand isobutyrene-isoprene copolymer rubber as a rubber ingredient andincluding closed cells in a matrix rubber, each of said cells beingcovered with a coat layer made from 2.5-20 parts by weight, based on 100parts by weight of the rubber ingredient, of a resin or resin compositehaving a JIS-C hardness of not less than 75, a particle size of 10-200μm and a reacted conjugate diene unit content of not less than 10% byweight.

According to a second aspect of the invention, there is the provision ofa method of producing a foamed rubber composition for pneumatic tires,which comprises adding a resin or resin composite having a JIS-Chardness of not less than 75, a particle size of 10-200 μm, a meltingpoint or glass transition point of 80°-200° C. and a reacted conjugatediene unit content of not less than 10% by weight to at least one dienepolymer selected from the group consisting of natural rubber, isoprenerubber, styrene-butadiene copolymer rubber, butadiene rubber andisobutyrene-isoprene copolymer rubber as a rubber ingredient togetherwith at least a foaming agent and a vulcanizing agent and kneading themat a temperature lower than the above melting point or glass transitionpoint while maintaining the above particle size; and vulcanizing theresulting kneaded mass at a temperature higher by at least 5° C. thanthe above melting point or glass transition point to cover each of theresulting closed cells with a coat layer made from the resin or resincomposite.

According to a third aspect of the invention, there is the provision ofa method of producing a foamed rubber composition for pneumatic tires,which comprises adding a resin or resin composite having a JIS-Chardness of not less than 75, a melting point or a glass transitionpoint of 80°-200° C. and a reacted conjugate diene unit content of notless than 10% by weight in form of block or pellet to at least one dienepolymer selected from the group consisting of natural rubber, isoprenerubber, styrene-butadiene copolymer rubber, butadiene rubber andisobutyrene-isoprene copolymer rubber as a rubber ingredient andkneading them at a temperature lower than the above melting point orglass transition point until a particle size of the resin or resincomposite is rendered into 10-200 μm; adding at least a foaming agentand a vulcanizing agent thereto and kneading them at a temperature lowerthan the above melting point or glass transition point while maintainingthe above particle size; and vulcanizing the resulting kneaded mass at atemperature higher by at least 5° C. than the above melting point orglass transition point to cover each of the resulting closed cells witha coat layer made from the resin or resin composite.

In preferable embodiments of the invention, the resin or resin compositeis made from at least one resin selected from the group consisting ofcrystalline syndiotactic-1,2-polybutadiene resin (hereinafterabbreviated as SPB resin simply) having a melting point of not lowerthan 80° C. and acrylonitrile-butadiene-styrene resin (hereinafterabbreviated as ABS resin simply) containing not less than 10% by weightof a reacted conjugate diene unit content and having a glass transitionpoint (Tg) of not lower than 80° C., and the resin composite contains0.3-5 parts by weight of sulfur and 0.1-7.0 parts by weight of avulcanization accelerator based on 100 parts by weight of the resin andfurther includes carbon black satisfying a relation of 0<X+10Y<2000wherein X is a nitrogen adsorption specific area of carbon black (m² /g)and Y is a compounding amount (parts by weight) of carbon black based on100 parts by weight of the resin, and the closed cells are existent at aclosed cell content of 5-35%.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, the resin or resin composite havingparticular hardness, melting point or glass transition point andco-crosslinking property with the matrix rubber is kneaded at aparticular kneading temperature with an unvulcanized rubber compositionto maintain at a state of dispersing at a particular particle size justbefore vulcanization, which is then vulcanized at a particularvulcanizing temperature to form a foamed rubber composition containingclosed cells therein, each of these cells being covered with a coatlayer made from the resin or resin composite melted during thevulcanization. As a result, the foamed rubber composition according tothe invention has an effect of largely improving the frictioncoefficient on an ice road surface of wet state.

As the rubber ingredient in the foamed rubber composition according tothe invention, there is used at least one diene polymer usually used ina tread of a pneumatic tire, which includes natural rubber, isoprenerubber, styrene-butadiene copolymer rubber, butadiene rubber,isobutyrene-isoprene copolymer rubber and the like.

The reason why the JIS-C hardness is defined in the resin or resincomposite according to the invention is due to the fact that the JIS-Chardness can not directly be measured in the coat layer made from theresin or resin composite because the closed cell covered with the coatlayer is too small and the coat layer itself is considered to have thesame JIS-C hardness as in the resin or resin composite. That is, theresin or resin composite is first melted during the vulcanization andthe closed cells are drawn in a portion of the melted resin or resincomposite, whereby the closed cell is encapsulated in a coat layer madefrom the melted resin or resin composite. When the JIS-C hardness isless than 75, the difference of hardness between the coat layer and ice(JIS-C hardness at 0° C.: 70) becomes less and hence the sufficientscratching effect by the coat layer can not be obtained.

In the resin or resin composite, the reason why the melting point orglass transition point of the resin is restricted to not lower than 80°C. is due to the fact that when the melting point or glass transitionpoint is lower than 80° C., the resin or resin composite is easilymelted and dispersed into rubber at usual kneading, warming-up andextrusion steps and hence it is very difficult to cover each of closedcells produced at the subsequent vulcanization step. Moreover, themelting point or glass transition point is preferable to be not higherthan 200° C. because the vulcanization is generally carried out at atemperature of 100°-200° C. If the melting point or glass transitionpoint exceeds 200° C., the resin or resin composite can not be meltedduring the vulcanization and hence the closed cell can not be coveredwith the coat layer made from the resin or resin composite. Moreover,when the vulcanization temperature exceeds 200° C., the basicperformances of the tire such as durability and the like are undesirablydegraded, so that the vulcanization for the tire is carried out at atemperature of not higher than 200° C.

The reason why the particle size of the resin or resin composite islimited to a range of 10-200 μm is due to that fact that when theparticle size is less than 10 μm, the coat layer is not satisfactorilyformed and the closed cell can not be covered with the coat layer, whilewhen it exceeds 200 μm, the closed cells are largely gathered in thecoat layer created during the vulcanization to form a big closed cellhaving a fairly large diameter after the vulcanization, which serves asa breaking nucleus to degrade the wear resistance and resistance tocracking at groove bottom.

When the reacted conjugate diene unit content in the resin or resincomposite is less than 10% by weight, the co-crosslinking property tothe matrix rubber is too lacking and the adhesion of the coat layer tothe matrix rubber is poor even if the closed cell can be covered withthis coat layer. Therefore, if such a foamed rubber composition isapplied to a tread of a pneumatic tire, the coat layer peels off from aworn surface of the tread during the running of the tire and hence thescratching effect of the coat layer is lost and the desired performanceson ice can not be obtained.

In the resin composite, when the amount of sulfur is less than 0.3 partby weight based on 100 parts by weight of the resin, the adhesion forceof the resulting coat layer to the matrix rubber after the vulcanizationis poor, while when it exceeds 5.0 parts by weight, the operability inthe production of the resin composite is considerably lowered. On theother hand, when the amount of the vulcanization accelerator is lessthan 0.1 part by weight, the adhesion force of the coat layer is poor,while when it exceeds 7.0 parts by weight, the operability isunfavorably lowered.

When a particular carbon black is compounded in the resin composite soas to satisfy the equation of 0<X+10Y<2000 defined in the invention, theJIS-C hardness of the resin composite is increased to improve thescratching effect of the resulting coat layer on ice. However, when thevalue of the equation is not less than 2000, the kneading operation isdifficult in the production of the resin composite. Moreover, the aboveequation is an empirical formula obtained by plotting experimentalresults on various properties and compounding amounts.

In the foamed rubber composition according to the invention, the reasonwhy the closed cell content is limited to a range of 5-35% is due to thefact that when the closed cell content is less than 5%, the waterdischarging effect of the matrix rubber containing the closed cellsbecomes insufficient on ice road and also the scratching action of thecoat layer on ice is not sufficient, while when it exceeds 35%, thedurability and wear resistance of the matrix rubber are undesirablydegraded. Particularly, the presence of such closed cells isindispensable to make large a microscopically water discharging effectat a state of existing a great amount of water on ice surface of about0° C. to develop the excellent performances on snow and ice surfaces.

As a reinforcement for the foamed rubber composition, silica and/orcarbon black may be compounded in a total amount of 10-100 parts byweight based on 100 parts by weight of the rubber ingredient.Furthermore, the foamed rubber composition may include additives usuallyused in the production of the tread rubber such as antioxidant,vulcanizing agent, vulcanization accelerator, accelerator activator andthe like.

In the production of the foamed rubber composition according to theinvention, the resin or resin composite satisfying the JIS-C hardnessand melting point to glass transition point defined in the invention isfirst added to and kneaded with the rubber ingredient. In order to formthe desired coat layer around the closed cell, it is necessary that theresin or resin composite is dispersed into the rubber ingredient at aparticle size of 10-200 μm after the kneading and before thevulcanization. For this purpose, the particulate resin or resincomposite having a particle size of 10-200 μm is directly added to therubber ingredient, or the pellet or block of the resin or resincomposite is added to the rubber ingredient and then finely divided intothe desired particle size in the kneading. In the latter case, it isrequired to severely control the kneading conditions for avoiding theexcessive kneading.

In order to maintain the particle size at the above range before thevulcanization and avoid the further decrease of the particle sizethrough the kneading, the temperature at the kneading step is limited tolower than the melting point or glass transition point of the resin.Then, the kneaded mass is vulcanized at a temperature higher by at least5° C. than the melting point or glass transition point of the resin,whereby the resin or resin composite is melted at a boundary to thematrix rubber and the closed cell is covered with the coat layer madefrom the molten resin or resin composite after the vulcanization. If thedifference between the vulcanizing temperature and the melting point orglass transition point is lower than 5° C., the melting of the resinbecomes insufficient during the vulcanization and the covering of theclosed cell with the coat layer is ununiform.

As a foaming agent for the formation of the closed cell in the foamedrubber composition according to the invention, use may be made ofazodicarbonamide, dinitrosopentamethylene tetramine,azobisisobutyronitrile, and aromatic sulfonylhydrazides such as benzenesulfonylhydrazide, toluene sulfonylhydrazide, oxybisbenzenesulfonylhydrazide and the like.

When the foamed rubber composition according to the invention is appliedto a tread of a pneumatic tire, the friction coefficient on ice isenhanced at not only dry-on-ice and wet-on-ice states and there can beobtained not only the same drainage effect as in the conventional foamedrubber but also the scratching effect by the coat layer covering theclosed cell, so that there can be provided pneumatic tires havingexcellent performances on ice and wear resistance. Furthermore, theclosed cell is covered with the coat layer strongly adhered to thematrix rubber, so that the formation and growth of microscopic cracksfrom the closed cell can be suppressed to improve the wear resistanceand durability.

The foamed rubber composition according to the invention is applied to atread in pneumatic tires, particularly pneumatic radial tires forpassenger cars, truck and bus and heavy duty vehicles used on snow andice road surfaces, whereby the excellent running performances on snowand ice roads are stably obtained over a wider temperature range.Further, the foamed rubber composition may be applied to other rubberarticles such as shoe sole, caterpillar and the like.

The following examples are given in illustration of the invention andare not intended as limitations thereof.

EXAMPLES 1-12, Comparative Examples 1-7

Various foamed rubber compositions are prepared through kneading andvulcanization according to a compounding recipe and conditions as shownin Table 1. Details of a resin or a resin composite used in theseexamples are shown in Table 2. In Table 1, the kneading temperature atthe compounding of the resin or resin composite means a temperature whenthe resin or resin composite is added to the unvulcanized rubbercomposition and dispersed thereinto through kneading, and the theworking temperature after the kneading means a temperature when thefoaming agent, vulcanizing agent and the like are dispersed into thekneaded mass containing the resin or resin composite through kneadingand warming-up, and the resin particle size before vulcanization means aparticle size of the resin or resin composite just before thevulcanization. Further, a test tire having a tire size of 165SR13 ismanufactured by using this foamed rubber composition in a tire tread.

The properties of the foamed rubber composition and the tire in Table 1are measured by the following methods, respectively.

Closed cell content

The closed cell content is represented by the following equation (1):

    V.sub.s ={(ρ.sub.0 -ρ.sub.g)/(ρ.sub.1 -ρ.sub.g)-1}×100 (%)                            (1)

wherein ρ₁ is a density of foamed rubber (g/cm³), ρ₀ is a density ofsolid phase portion in the foamed rubber (g/cm³), and ρ_(g) is a densityof gas portion in the foamed rubber (g/cm³). The foamed rubber iscomposed of the solid phase portion and the gas portion formed in thesolid phase portion as a cavity (closed cell). Therefore, the densityρ_(g) of the gas portion is very small and is approximately near tozero, so that the equation (1) is approximated to the following equation(2):

    V.sub.s ={(ρ.sub.0 /ρ.sub.1)-1 }×100 (%)     (2)

Closed cell size and particle size of resin or resin composite beforevulcanization

The mean closed cell size or the particle size of the resin or resincomposite before vulcanization is measured by cutting out a blockspecimen from a sample rubber composition and photographing a surface ofthe specimen by means of an optical microscope of 100-400 magnificationsto measure sizes of 200 or more closed cells or 200 or more resinparticle sizes of the resin or resin composite and represented as anarithmetical mean value.

Percentage of closed cells covered

The percentage of closed cells covered is calculated by observing thecut surface of the specimen by means of the optical microscope to countthe number of closed cells covered to the total closed cell number per 1mm² of the cut surface.

Friction coefficient on ice

The test tire is trained straight forwards on an asphalt paved road overa distance of 200 km and then subjected to braking from a speed of 20km/h at an ice temperature of -2° C. or -8° C. to measure a brakingdistance. The friction coefficient on ice is represented by a reverseindex of the measured braking distance on the basis that ComparativeExample 1 or 4 is 100. The larger the index value, the better thebraking property on ice.

Wear resistance

The test tire is actually run on a concrete road over a given distanceand then the remaining groove depth is measured. The wear resistance isrepresented by an index on the basis that the measured groove depth inComparative Example 1 or 4 is 100. The larger the index value, thebetter the wear resistance.

Appearance

The test tire is actually run on a circuit course over a distance of7000 km and thereafter the surface of the tread rubber is observed onthe circumference of the tire. The cracking appearance is evaluated as⊚: no crack, ∘: crack length of not more than 0.5 mm, Δ: crack length ofmore than 0.5 mm but not more than 1 mm, and X: crack length of morethan 1 mm.

Dynamic modulus (E' at -20° C.)

The dynamic modulus at -20° C. is measured by attaching a sample of 20mm in length, 4.7 mm in width and 2 mm in thickness to a viscoelasticitymeasuring device (L-IR model, made by Toyo Seiki Kabushiki Kaisha) andapplying an initial tension of 1.7 kgf/cm², an amplitude strain of 1%and a frequency of 50 Hz thereto.

                                      TABLE 1                                     __________________________________________________________________________                                       Compar-          Compar-                                                                            Compar-                                                 ative            ative                                                                              ative                (part by weight)                                                                           Example 1                                                                           Example 2                                                                          Example 3                                                                           Example 4                                                                          Example 1                                                                           Example 5                                                                          Example 6                                                                           Example                                                                            Example              __________________________________________________________________________                                                             3                    NR           70    70   70    70   70    70   70    70   70                   BR           30    30   30    30   30    30   30    30   30                   SBR          0     0    0     0    0     0    0     0    0                    Carbon black N220                                                                          45    45   45    45   45    45   45    45   45                   Stearic acid 2.0   2.0  2.0   2.0  2.0   2.0  2.0   2.0  2.0                  ZnO          4.5   4.5  4.5   4.5  4.5   4.5  4.5   4.5  4.5                  Antioxidant  1.0   1.0  1.0   1.0  1.0   1.0  1.0   1.0  1.0                  Accel DM*1   0.3   0.3  0.3   0.3  0.3   0.3  0.3   0.3  0.3                  Accel CZ*2   0.6   0.6  0.6   0.6  0.6   0.6  0.6   0.6  0.6                  Sulfur       1.2   1.2  1.2   1.2  1.2   1.2  1.2   1.2  1.2                  ADCA*3/urea*4                                                                              4.8   4.8  4.8   4.8  5.3   4.4  7.2   3.8  4.8                  Resin or resin composite                                                      JIS-C hardness                                                                             82    94   92    97   --    82   82    82   92                   kind         A     H    C     D    --    A    A     A    E                    amount (part by weight)                                                                    7.0   7.0  7.0   7.0  --    10.0 15.0  25.0 7.0                  Tg or mp. (°C.)                                                                     140   143  110   140  --    140  140   140  115                  particle size (μm)                                                                      40    40   40    40   --    --   --    --   40                   Kneading temperature at the                                                                100   100  100   100  --    100  100   100  100                  compounding of resin or                                                       resin composite (°C.)                                                  Working temperature after                                                                  100   100  100   100  100   100  100   100  100                  the kneading (°C.)                                                     Resin particle size before                                                                 40    40   40    40   --    40   40    40   40                   vulcanization (μm)                                                         Vulcanizing temperature (°C.)                                                       145   145  145   145  145   145  145   145  145                  Closed cell content (%)                                                                    16    16   16    16   16    16   16    16   16                   Closed cell size (μm)                                                                   65    65   65    65   65    65   65    65   65                   Percentage of closed cell                                                                  83    85   83    83   0     94   100   100  85                   covered (%)                                                                   Index of friction                                                                          130   138  132   141  100   134  137   132  105                  coefficient on ice (-2° C.)*5                                          Index of friction                                                                          111   113  111   118  100   112  114   111  102                  coefficient on ice (-8° C.)*5                                          Wear resistance (index)                                                                    103   103  102   101  100   100  100   90   98                   Appearance   ⊚                                                                    ⊚                                                                   ⊚                                                                    ⊚                                                                   ◯                                                                       ◯                                                                      ◯                                                                       Δ                                                                            ◯        E' at -20° C.                                                                       145   145  145   145  140   145  145   155  145                  (×10.sup.6 dyn/cm.sup.2)                                                __________________________________________________________________________                                     Compar-                                                                            Compar-                                                                            Compar-        Compar-                                         Example                                                                            ative                                                                              ative                                                                              ative                                                                              Example                                                                            Example                                                                            ative               (part by weight)                                                                           Example 7                                                                          Example 8                                                                          Example 9                                                                          10   Example 4                                                                          Example 5                                                                          Example 6                                                                          11   12   Example             __________________________________________________________________________                                                              7                   NR           50   50   50   50   50   50   50   50   50   50                  BR           30   30   30   30   30   30   30   30   30   30                  SBR          20   20   20   20   20   20   20   20   20   20                  Carbon black N220                                                                          50   50   50   50   50   50   50   50   50   50                  Stearic acid 3.0  3.0  3.0  3.0  3.0  3.0  3.0  3.0  3.0  3.0                 ZnO          4.0  4.0  4.0  4.0  4.0  4.0  4.0  4.0  4.0  4.0                 Antioxidant  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0                 Accel DM*1   0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2                 Accel CZ*2   0.5  0.5  0.5  0.5  0.5  0.5  0.5  0.5  0.5  0.5                 Sulfur       1.2  1.2  1.2  1.2  1.2  1.2  1.2  1.2  1.2  1.2                 ADCA*3/urea*4                                                                              5.0  5.0  5.0  5.0  5.2  4.5  5.0  5.0  4.4  4.5                 Resin or resin composite                                                      JIS-C hardness                                                                             82   94   92   97   --   74   94   94   94   94                  kind         A    B    C    D    --   F    G    H    I    J                   amount (part by weight)                                                                    7.5  7.5  7.5  7.5  --   7.5  7.5  7.5  7.5  7.5                 Tg or mp. (°C.)                                                                     140  143  110  140  --   110  143  143  143  143                 particle size (μm)                                                                      40   40   40   40   --   40   5.0  10.0 200  240                 Kneading temperature at the                                                                100  100  100  100  --   100  100  100  100  100                 compounding of resin or                                                       resin composite (°C.)                                                  Working temperature after                                                                  100  100  100  100  100  100  100  100  100  100                 the kneading (°C.)                                                     Resin particle size before                                                                 40   40   40   40   --   40   5.0  10.0 200  240                 vulcanization (μm)                                                         Vulcanizing temperature (°C.)                                                       155  155  155  155  155  155  155  155  155  155                 Closed cell content (%)                                                                    22   22   22   22   22   22   22   22   22   22                  Closed cell size (μm)                                                                   75   75   75   75   75   75   75   75   380  720                 Percentage of closed cell                                                                  81   81   81   81   0    81   18   74   80   80                  covered (%)                                                                   Index of friction                                                                          124  131  128  137  100  100  100  118  146  146                 coefficient on ice (-2° C.)*5                                          Index of friction                                                                          109  111  110  113  100  100  100  106  126  124                 coefficient on ice (-8° C.)*5                                          Wear resistance (index)                                                                    103  102  104  102  100  100  96   100  100  86                  Appearance   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ◯                                                                      ◯                                                                      Δ                                                                            ◯                                                                      ◯                                                                      X                   E' at -20° C.                                                                       165  165  165  165  160  165  165  165  165  165                 (×10.sup.6 dyn/cm.sup.2)                                                __________________________________________________________________________     *1: dibenzothiazyl disulfide (vulcanization accelerator)                      *2: Ncyclohexyl-2-benzothiazyl sulfenamide (vulcanization accelerator)        *3: dinitrosopentamethylene tetraamine                                        *4: mixing ratio of DPT/urea = 1:1 (weight ratio)                             *5: dryon-ice state at -8° C., weton-ice state at -2° C.   

                                      TABLE 2                                     __________________________________________________________________________                                           Reacted                                Kind of                                conjugate                              resin or                           Particle                                                                          diene unit                             resin   Tg or mp.                                                                          Carbon black  Accel                                                                            Scorch                                                                             size                                                                              content                                                                            JIS-C                             composite                                                                          Resin                                                                            (°C.)                                                                       X Y  X + 10Y                                                                            Sulfur                                                                            NS*6                                                                             reterdar*7                                                                         (μm)                                                                           (wt %)                                                                             hardness                          __________________________________________________________________________    A    SPB                                                                              140  --                                                                              -- --   --  -- --   40  100  82                                B    ABS                                                                              143  --                                                                              -- --   --  -- --   40  12   94                                C    RB 110  83                                                                              100                                                                              1083 0.5 0.5                                                                              0.5  40  50   92                                D    SPB                                                                              140  83                                                                              50 583  0.5 0.5                                                                              0.5  40  66   97                                E    AS 115  --                                                                              -- --   --  -- --   40  0    92                                F    RB 110  83                                                                              25 283  0.5 0.5                                                                              0.5  40  79   74                                G    ABS                                                                              143  --                                                                              -- --   --  -- --   5.0 12   94                                H    ABS                                                                              143  --                                                                              -- --   --  -- --   10  12   94                                I    ABS                                                                              143  --                                                                              -- --   --  -- --   200 12   94                                J    ABS                                                                              143  --                                                                              -- --   --  -- --   240 12   94                                __________________________________________________________________________     *6: Noxydiethylene-2-benzothiazolylsulfenamide                                *7: Santoguard PV125, trade name made by Nippon Monsanto Company Limited 

Company Limited

Comparative Examples 1 and 4 are so-called conventional foamed rubbercontaining closed cells not covered with the coat layer, so that theyare low in the friction coefficient on ice as compared with those of theexamples according to the invention. On the contrary, the frictioncoefficient on ice is largely improved in all examples containing closedcells covered with the coat layer.

In Comparative Example 2, the closed cell is covered with the coatlayer, but the amount of the resin is too large, so that the wearresistance is degraded. This shows that it is necessary to maintain theamount of the resin at a given range for improving the wear resistance.

In Comparative Example 3, the conjugate diene monomer is not used in theproduction of the resin, so that the crosslinking property to the matrixrubber and the wear resistance are poor. Further, the scratching effectis small and the improvement of performances on ice is slight.

In Comparative Example 5, the JIS-C hardness of the resin constitutingthe coat layer is outside the range defined in the invention, so thatthe scratching effect of the coat layer is not obtained and issubstantially equal to that of the conventional foamed rubber. Thisshows that the coat layer is necessary to have a JIS-C hardness of agiven value or more for obtaining the satisfactory scratching effect.

In Comparative Example 6, the particle size of the resin in thevulcanization is smaller than the range defined in the invention, sothat a greater amount of closed cells are not covered with the coatlayer and the satisfactory scratching effect is not obtained. InComparative Example 7, the particle size of the resin in thevulcanization is larger than the range defined in the invention, so thatthe size of the closed cell covered becomes larger and hence the wearresistance is degraded. This shows that the particle size of the resinor resin composite in the foamed rubber is important to be maintained ata given range for effectively covering the closed cell.

EXAMPLES 13-22, Comparative Examples 8-15

Various foamed rubber compositions are prepared through kneading andvulcanization according to a compounding recipe and conditions as shownin Table 3 or 4. Details of a resin or a resin composite used in theseexamples are shown in Table 5. Further, a test tire having a tire sizeof 165SR13 is manufactured by using this foamed rubber composition in atire tread.

Concretely, the foamed rubber composition is produced as follows:

The unvulcanized rubber composition is kneaded at two stages in whichpolymer, carbon black, stearic acid, ZnO, antioxidant and the like arekneaded in a first stage and sulfur, vulcanization accelerator, foamingagent and the like are kneaded with the resulting kneaded mass of thefirst stage in a second stage.

The kneading is carried out in an OOC Banbury mixer at a startingtemperature of 70° C. and a revolution number of 80 rpm under a rampressure of 5 kg/cm² till the kneading temperature reaches to a givenvalue (kneading temperature at the compounding of resin or resincomposite shown in Table 3).

The time of compounding the resin or resin composite at either of thefirst stage and the second stage is determined by the shape of the resinor resin composite (particle, pellet, block or the like).

In case of the pellet or block, the resin or resin composite is added atthe first stage and kneaded at a temperature higher than the meltingpoint or glass transition point of the resin or resin composite butlower than the vulcanizing temperature so as to disperse at aparticulate state having a given particle size range.

When the resin or resin particle has previously a given particle size,it is only added at the second stage.

The properties of the foamed rubber and the tire shown in Tables 3 and 4are measured by the same methods as in Example 1.

                                      TABLE 3                                     __________________________________________________________________________                    Compar-                                                                            Compar-                                                                            Compar-                                                                            Compar-  Compar-                                                                            Compar-                                                                            Compar-                                     ative                                                                              ative                                                                              ative                                                                              ative    ative                                                                              ative                                                                              ative                                   Exam-                                                                             Example                                                                            Example                                                                            Example                                                                            Example                                                                            Exam-                                                                             Example                                                                            Example                                                                            Example                                                                            Exam-                                                                             Exam-              (part by weight)                                                                          ple 13                                                                            8    9    10   11   ple 14                                                                            12   13   14   ple                                                                               ple                __________________________________________________________________________                                                               16                 NR          70  70   70   70   70   70  70   70   70   70  70                 BR          30  30   30   30   30   30  30   30   30   30  30                 Carbon black N220                                                                         50  50   50   50   50   50  50   50   50   50  50                 Stearic acid                                                                              1.5 1.5  1.5  1.5  1.5  1.5 1.5  1.5  1.5  1.5 1.5                ZnO         4.0 4.0  4.0  4.0  4.0  4.0 4.0  4.0  4.0  4.0 4.0                Antioxidant 1.0 1.0  1.0  1.0  1.0  1.0 1.0  1.0  1.0  1.0 1.0                Accel DM*1  0.2 0.2  0.2  0.2  0.2  0.2 0.2  0.2  0.2  0.2 0.2                Accel CZ*2  0.5 0.5  0.5  0.5  0.5  0.5 0.5  0.5  0.5  0.5 0.5                Sulfur      1.2 1.2  1.2  1.2  1.2  1.2 1.2  1.2  1.2  1.2 1.2                ADCA*3/urea*4                                                                             5.2 3.8  5.2  5.2  5.4  5.2 5.2  5.2  5.2  4.8 4.2                Resin or resin composite                                                      kind        A   A    A    A    --   B   B    B    B    A   A                  amount (part by weight)                                                                   7.5 7.5  7.5  7.5  --   7.5 7.5  7.5  7.5  7.5 7.5                Tg or mp. (°C.)                                                                    130 130  130  130  --   130 130  130  130  130 130                shape or    block                                                                             block                                                                              block                                                                              block                                                                              --   40  40   40   40   block                                                                             block              particle size (μm)                                                         Kneading temperature at                                                                   140 130  140  140  --   100 130  100  100  135 133                the compounding of resin                                                      or resin composite (°C.)                                               Working temperature after                                                                 120 120  135  120  --   120 120  135  120  120 120                the kneading (°C.)                                                     Resin particle size before                                                                35  220  4.5  35   --   40  8.0  6.5  40   60  95                 vulcanization (μm)                                                         Vulcanizing temperature                                                                   135 135  135  125  135  135 135  135  120  135 135                (°C.)                                                                  Closed cell content (%)                                                                   20  20   20   20   20   20  20   20   20   20  20                 Closed cell size (μm)                                                                  70  1300 70   70   70   70  70   70   70   105 150                Percentage of closed cell                                                                 80  80   14   0    0    80  16   14   0    85  98                 covered (%)                                                                   Index of friction                                                                         135 125  100  105  100  139 100  100  108  137 132                coefficient on ice                                                            (-2° C.)*5                                                             Index of friction                                                                         118 108  100  100  100  119 100  100  100  114 113                coefficient on ice                                                            (-8° C.)*5                                                             Wear resistance (index)                                                                   105 50   100  90   100  102 100  100  86   103 102                Appearance  ⊚                                                                  X    X    X    ◯                                                                      ⊚                                                                  X    X    X    ⊚                                                                  ◯      E' at -20° C.                                                                      145 145  145  140  140  145 145  145  140  145 145                (×10.sup.6 dyn/cm.sup.2)                                                __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                                         Comparative                              (part by weight)                                                                           Example 17                                                                          Example 18                                                                          Example 19                                                                          Example 20                                                                          Example 15                                                                          Example 21                                                                          Example                      __________________________________________________________________________                                                     22                           NR           70    70    70    70    70    70    70                           BR           30    30    30    30    30    30    30                           Carbon black N220                                                                          50    50    50    50    50    50    50                           Stearic acid 1.5   1.5   1.5   1.5   1.5   1.5   1.5                          ZnO          4.0   4.0   4.0   4.0   4.0   4.0   4.0                          Antioxidant  1.0   1.0   1.0   1.0   1.0   1.0   1.0                          Accel DM*1   0.2   0.2   0.2   0.2   0.2   0.2   0.2                          Accel CZ*2   0.5   0.5   0.5   0.5   0.5   0.5   0.5                          Sulfur       1.2   1.2   1.2   1.2   1.2   1.2   1.2                          ADCA*3/urea*4                                                                              5.0   4.4   3.8   4.0   5.2   5.2   4.2                          Resin or resin composite                                                      kind         C     C     C     D     E     F     G                            amount (part by weight)                                                                    2.5   7.5   15    7.5   7.5   7.5   7.5                          Tg or mp. (°C.)                                                                     140   140   140   130   130   130   130                          particle size (μm)                                                                      80    80    80    110   8.0   15    30                           Kneading temperature at the                                                                100   100   100   l00   100   100   120                          compounding of resin or                                                       resin composite (°C.)                                                  Working temperature after                                                                  120   120   120   120   120   120   120                          the kneading (°C.)                                                     Resin particle size before                                                                 80    80    80    110   8.0   15.0  30.0                         vulcanization (μm)                                                         Vulcanizing temperature (°C.)                                                       145   145   145   135   135   135   135                          Closed cell content (%)                                                                    20    20    20    20    20    20    20                           Closed cell size (μm)                                                                   130   130   130   170   70    70    70                           Percentage of closed cell                                                                  22    80    98    80    14    24    82                           covered (%)                                                                   Index of friction                                                                          104   119   120   121   100   109   134                          coefficient on ice (-2° C.)*5                                          Index of friction                                                                          100   107   110   110   100   103   114                          coefficient on ice (-8° C.)*5                                          Wear resistance (index)                                                                    100   100   100   100   100   100   104                          Appearance   ◯                                                                       ⊚                                                                    ◯                                                                       ◯                                                                       X     ⊚                                                                    ⊚             E' at -20° C.                                                                       140   140   150   145   145   145   145                          (×10.sup.6 dyn/cm.sup.2)                                                __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                                       Shape                                                                             Reacted                                Kind of                            or  conjugate                              resin or                           Particle                                                                          diene unit                             resin   Tg or mp.                                                                          Carbon black  Accel                                                                            Scorch                                                                             size                                                                              content                                                                            JIS-C                             composite                                                                          Resin                                                                            (°C.)                                                                       X Y  X + 10Y                                                                            Sulfur                                                                            NS*6                                                                             reterdar*7                                                                         (μm)                                                                           (wt %)                                                                             hardness                          __________________________________________________________________________    A    SPB                                                                              130  83                                                                              100                                                                              1083 0.5 0.5                                                                              0.5  block                                                                             50   96                                B    ABS                                                                              130  --                                                                              -- --   --  -- --   40  14   94                                C    SPB                                                                              130  --                                                                              -- --   --  -- --   80  100  84                                D    SPB                                                                              130  83                                                                              50 583  --  -- --   110 67   89                                E    SPB                                                                              130  83                                                                              50 583  --  -- --   8.0 67   89                                F    SPB                                                                              130  83                                                                              50 583  --  -- --   15.0                                                                              67   89                                G    SPB                                                                              130  --                                                                              -- --   --  -- --   30  100  89                                __________________________________________________________________________

In Comparative Example 8, the particle size of the resin compositebefore the vulcanization is larger than a given value defined in theinvention, so that the size of closed cell is too large and the wearresistance and appearance are poor. On the other hand, the wearresistance and appearance are excellent in all examples because theparticle size before the vulcanization is within a given range definedin the invention.

In Comparative Example 9, the working temperature after the kneading ishigher than the melting point of the resin, so that the dispersion ofthe resin composite through melting is promoted and the particle sizebefore the vulcanization is too small, and hence the number of closedcells covered with the coat layer becomes small and the scratchingeffect is not sufficiently obtained and the friction coefficient on iceis not improved. On the other hand, the friction coefficient on ice isimproved in all examples because the coat layer is formed to an extentof sufficiently providing the scratching effect. As the percentage ofclosed cell covered becomes higher, the friction coefficient on icebecomes higher.

In Comparative Examples 10 and 14, the vulcanizing temperature is lowerthan the melting point of the resin, so that the resin composite can notbe melted during the vulcanization for covering the closed cell andhence the wear resistance is degraded. This shows that the vulcanizationtemperature is important to be not lower than the melting point of theresin for covering the closed cell.

Comparative Example 11 is a foamed rubber containing no resin or resincomposite prepared as a control for evaluation of friction coefficienton ice and wear resistance.

In Comparative Example 12, the resin having a given particle size iskneaded with the unvulcanized rubber composition at the melting pointthereof, so that the particle size of the resin is rendered into a valuesmaller than the given value during the kneading and hence the amount ofthe coated closed cell is small and the friction coefficient on ice isnot improved. This shows that it is important to maintain the particlesize of the resin within a given range before the vulcanization.

In Comparative Example 13, the working temperature after the kneadingexceeds the melting point of the resin, so that the melting anddispersion of the resin is too promoted and hence the formation of thecoat layer is insufficient.

In Comparative Example 15, the particle size of the resin composite isless than a given value at the compounding stage before thevulcanization, so that the covering of the closed cell is insufficientand the satisfactory results are not obtained.

EXAMPLES 23-34, Comparative Examples 16-24

Various foamed rubber compositions are prepared through kneading andvulcanization according to a compounding recipe and conditions as shownin Table 6. Details of a resin or a resin composite used in theseexamples are shown in Table 7. Further, a test tire having a tire sizeof 165SR13 is manufactured by using this foamed rubber composition in atire tread.

Since the particle size of the resin or resin composite used in theseexamples is within a range defined in the invention, the resin or resincomposite is added at the second stage in the same two-stage kneading asin Example 13.

The properties of the foamed rubber and the tire shown in Table 6 aremeasured by the same methods as in Example 1.

                                      TABLE 6                                     __________________________________________________________________________                                   Compar-           Compar-                                                                            Compar-                                                ative             ative                                                                              ative                                Exam-                                                                              Exam-                                                                             Exam-                                                                              Exam-                                                                             Example                                                                            Exam-                                                                             Exam-                                                                              Exam-                                                                             Example                                                                            Example                                                                            Exam-              (part by weight)                                                                           ple 23                                                                             ple 24                                                                            ple 25                                                                             ple 26                                                                            17   ple 27                                                                            ple 28                                                                             ple 29                                                                            18   19   ple                __________________________________________________________________________                                                               30                 NR           60   60  60   60  60   60  60   60  60   60   60                 BR           40   40  40   40  40   40  40   40  40   40   40                 Carbon black N220                                                                          50   50  50   50  55   50  50   50  50   54   53                 Stearic acid 1.5  1.5 1.5  1.5 1.5  1.5 1.5  1.5 1.5  1.5  1.5                ZnO          3.0  3.0 3.0  3.0 3.0  3.0 3.0  3.0 3.0  3.0  3.0                Antioxidant  1.0  1.0 1.0  1.0 1.0  1.0 1.0  1.0 1.0  1.0  1.0                Accel DM*1   0.2  0.2 0.2  0.2 0.2  0.2 0.2  0.2 0.2  0.2  0.2                Accel CZ*2   0.5  0.5 0.5  0.5 0.5  0.5 0.5  0.5 0.5  0.5  0.5                Sulfur       5.0  5.0 5.0  5.0 5.2  5.0 5.0  5.0 5.0  5.0  5.0                ADCA*3/urea*4                                                                              1.1  1.1 1.1  1.1 1.1  1.1 1.1  1.1 1.1  1.1  1.1                Resin or resin composite                                                                   97   7g  82   92  --   89  92   94  92   97   97                 JIS-C hardness                                                                             A    B   C    D   --   E   F    G   H    A    A                  kind         7.5  7.5 7.5  7.5 --   7.5 7.5  7.5 7.5  2    2.5                amount (part by weight)                                                                    140  140 140  140 --   102 119  143 115  140  140                Tg or mp. (°C.)                                                        Kneading temperature at the                                                                95   95  95   95  95   95  95   95  95   95   95                 compounding of resin or                                                       resin composite (°C.)                                                  Working temperature after                                                                  95   95  95   95  95   95  95   95  95   95   95                 the kneading (°C.)                                                     Vulcanizing temperature (°C.)                                                       150  150 150  150 150  150 150  150 150  150  150                Closed cell content (%)                                                                    20   20  20   20  20   20  20   20  20   20   20                 Closed cell size (μm)                                                                   65   65  65   65  65   65  65   65  65   65   65                 Percentage of closed cell                                                                  80   80  80   80  80   80  80   80  80   10   22                 covered (%)                                                                   Index of friction                                                                          125  104 108  114 100  128 127  124 100  100  108                coefficient on ice (-2° C.)*5                                          Index of friction                                                                          112  102 105  108 100  113 113  112 100  100  103                coefficient on ice (-8° C.)*5                                          Wear resistance (index)                                                                    102  101 101  102 100  102 100  100 100  100  100                Appearance   ⊚                                                                   ⊚                                                                  ⊚                                                                   ⊚                                                                  ⊚                                                                   ⊚                                                                  ⊚                                                                   ⊚                                                                  ⊚                                                                   ⊚                                                                   .circleincircle                                                               .                  E' at -20° C.                                                                       90   90  90   90  90   90  90   90  90   90   90                 (×10.sup.6 dyn/cm.sup.2)                                                __________________________________________________________________________                      Compar-                                                                            Compar-   Compar-                                                                            Compar-   Compar-   Compar-                               ative                                                                              ative     ative                                                                              ative     ative     ative                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example             (part by weight)                                                                           31   20   16   32   21   22   33   23   34   24                  __________________________________________________________________________    NR           60   60   60   60   60   60   60   60   60   60                  BR           40   40   40   40   40   40   40   40   40   40                  Carbon black N220                                                                          46   42   50   50   50   55   50   55   52   55                  Stearic acid 1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5  1.5                 ZnO          3.0  3.0  3.0  3.0  3.9  3.0  3.0  3.0  3.0  3.0                 Antioxidant  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0                 Accel DM*1   0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2  0.2                 Accel CZ*2   0.5  0.5  0.5  0.5  0.5  0.5  0.5  0.5  0.5  0.5                 Sulfur       1.1  1.1  1.1  1.1  1.1  1.1  1.1  1.1  1.1  1.1                 ADCA*3/urea*4                                                                              5.0  5.0  5.0  5.0  8.5  8.6  7.7  7.7  2.8  3.3                 Resin or resin composite                                                      JIS-C hardness                                                                             97   97   74   94   94   --   94   --   94   --                  kind         A    A    J    I    A    --   A    --   A    --                  amount (part by weight)                                                                    20   25   7.5  7.5  7.5  --   7.5  --   3    --                  Tg or mp. (°C.)                                                                     140  140  110  110  140  --   140  --   140  --                  Kneading temperature at the                                                                95   95   95   95   95   95   95   95   95   95                  compounding of resin or                                                       resin composite (°C.)                                                  Working temperature after                                                                  95   95   95   95   95   95   95   95   95   95                  the kneading (°C.)                                                     Vulcanizing temperature (°C.)                                                       150  150  150  150  150  150  150  150  150  150                 Closed cell content (%)                                                                    20   20   20   20   40   40   35   35   5    5                   Closed cell size (μm)                                                                   65   65   65   65   85   85   80   80   45   45                  Percentage of closed cell                                                                  100  100  80   80   65   --   72   --   100  --                  covered (%)                                                                   Index of friction                                                                          112  102  100  122  117  110  108  100  104  100                 coefficient on ice (-2° C.)*5                                          Index of friction                                                                          107  100  100  110  106  100  105  100  102  100                 coefficient on ice (-8° C.)*5                                          Wear resistance (index)                                                                    100  97   101  101  91   81   102  100  102  100                 Appearance   ⊚                                                                   ◯                                                                      ⊚                                                                   ⊚                                                                   ◯                                                                      Δ                                                                            ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                              5                   E' at -20° C.                                                                       90   90   90   90   65   65   72   72   125  125                 (×10.sup.6 dyn/cm.sup.2)                                                __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________                                           Reacted                                Kind of                                conjugate                              resin or                           Particle                                                                          diene unit                             resin   Tg or mp.                                                                          Carbon black  Accel                                                                            Scorch                                                                             size                                                                              content                                                                            JIS-C                             composite                                                                          Resin                                                                            (°C.)                                                                       X Y  X + 10Y                                                                            Sulfur                                                                            NS*6                                                                             reterdar*7                                                                         (μm)                                                                           (wt %)                                                                             hardness                          __________________________________________________________________________    A    SPB                                                                              140  83                                                                              50 583  0.5 0.3                                                                              0.3  40  66   97                                B    SPB                                                                              140  --                                                                              -- --   --  -- --   40  100  79                                C    SPB                                                                              140  --                                                                              -- --   0.5 0.3                                                                              0.3  40  100  82                                D    SPB                                                                              140  83                                                                              40 483  0.5 0.3                                                                              0.3  40  71   92                                E    ABS                                                                              102  --                                                                              -- --   --  -- --   40  25   89                                F    ABS                                                                              119  --                                                                              -- --   --  -- --   40  15   92                                G    ABS                                                                              143  --                                                                              -- --   --  -- --   40  12   94                                H    ABS                                                                              115  --                                                                              -- --   --  -- --   40  0    92                                I    SBP                                                                              110  83                                                                              100                                                                              1083 0.5 0.3                                                                              0.3  40  50   94                                J    SPB                                                                              110  83                                                                              25 283  0.5 0.3                                                                              0.3  40  79   74                                __________________________________________________________________________

As to the indexes of fiction coefficient on ice and wear resistance,Comparative Example 17 is control for Examples 23-32 and ComparativeExamples 16-22, and Comparative Example 23 is control for Example 33,and Comparative Example 24 is control for Example 34, respectively.

As seen from Examples 23-26 and Comparative Example 17, the frictioncoefficient on ice is improved as the JIS-C hardness of the resin orresin composite becomes higher. As seen from Examples 27-29 andComparative Example 18, the friction coefficient on ice and wearresistance are good as the reacted conjugate diene unit content becomeslarge.

As seen from Examples 27-28 and Comparative Examples 19-20, when theamount of the resin or resin composite compounded is small, the frictioncoefficient on ice is not improved, while when it is too large, the wearresistance lowers. From Example 32 and Comparative Example 16, it can beseen that the friction coefficient on ice is not improved when the JIS-Chardness of the resin composite is low.

From Comparative Examples 17, 21 and 22, it can be seen that when theclosed cell content is too high, the friction coefficient on ice isimproved but the wear resistance is degraded. When Example 33 iscompared with Comparative Example 23, the effect of improving thefriction coefficient on ice is observed at the same high closed cellcontent. Further, when Example 34 is compared with Comparative Example24, the effect of improving the friction coefficient on ice and wearresistance is observed at the same low closed cell content.

What is claimed is:
 1. A foamed rubber composition for pneumatic tirescomprising at least one diene polymer selected from the group consistingof natural rubber, isoprene rubber, styrene-butadiene copolymer rubber,butadiene rubber and isobutylene-isoprene copolymer rubber as a rubberingredient and including closed cells in a matrix rubber, each of saidcells being covered with a coat layer made from 2.5-20 parts by weight,based on 100 parts by weight of the rubber ingredient, of a resin orresin composite having a JIS-C hardness of not less than 75, a particlesize of 10-200 μm, and a reacted conjugate diene unit content of notless than 10% by weight and a glass transition point (Tg) of not lowerthan 80° C.
 2. A foamed rubber composition according to claim 1, whereinthe resin or resin composite is made from at least one resin selectedfrom the group consisting of crystalline syndiotactic-1,2-polybutadieneresin having a melting point of not lower than 80° C. andacrylonitrilebutadiene-styrene resin containing not less than 10% byweight of a reacted conjugate diene unit content and having a glasstransition point (Tg) of not lower than 80° C.
 3. A foamed rubbercomposition according to claim 1, wherein the resin composite contains0.3-5 parts by weight of sulfur and 0.1-7.0 parts by weight of avulcanization accelerator based on 100 parts by weight of the resin. 4.A foamed rubber composition according to claim 1, wherein the resincomposite further includes carbon black satisfying a relation of0<X+10Y<2000 wherein X is a nitrogen adsorption specific area of carbonblack (m² /g) and Y is a compounding amount (parts by weight) of carbonblack based on 100 parts by weight of the resin.
 5. A foamed rubbercomposition according to claim 1, wherein the closed cells are existentat a closed cell content of 5-35%.